1. Field of the Invention
The present invention relates to the rejection of electronic interference. More specifically, the present invention relates to a method and apparatus for digital interference rejection.
2. The Prior Art
Interference rejection is performed in many types of electronic systems. However, it has specific application in the process of tuning channels for television or radio equipment because of the fact that a specific channel the user wishes to tune in is surrounded on either side by interference (either other channels or noise). In recent years, digital television signals have been becoming more and more prevalent as more consumers have discovered the benefits (better picture resolution, clearer sound quality) of digital television. Digital satellite television has been one of the types of communication to benefit from this transition.
The original signal received by a satellite dish (sometimes called a dish antenna) comprises a large number of channels. The tuning process takes this signal and alters it so that the channel the user wishes to view is exactly at the midpoint of the signal. This tuning process varies by the type of implementation. Oftentimes, tuning is performed in several different steps, with filters mixed in between the tuning steps to narrow the bandwidth of the signal and make it more manageable. The prior art tuners generally perform the entire tuning process on analog signals. Therefore, the conversion from analog to digital signal would take place after the tuner has been encountered.
One of the advantages of digital satellite systems is the ability to have a variable bandwidth system. This allows each channel to have a different sized bandwidth if the provider so wishes. The advantage of this is that the provider could specify a large bandwidth for a channel that will contain a great deal of information (perhaps a movie channel, where picture quality and sound quality are most important), while specifying a small bandwidth for a channel that will contain less information (perhaps a news channel, or an audio-only channel) in order to efficiently allocate overall bandwidth and allow the provider to transmit a larger number of channels than would be possible with fixed bandwidth allocation. The difference between a fixed bandwidth system and a variable bandwidth system can be seen in FIGS. 1-2.
FIG. 1 shows an example of the channel spectrum in a fixed channel bandwidth system. The spectrum in such a system is predefined such that each channel uses the same amount of bandwidth. In FIG. 1, moving up and down along the vertical axis of the spectrum represents higher or lower frequencies. It is apparent that channels 10, 11, 12, and 13 all take up the same amount of bandwidth. These types of systems are still in use today in most radio and television broadcasts. One of their advantages is that a single receiver (the television or radio) can be used to tune in a multitude of different systems.
Currently, most television and radio stations are xe2x80x9clocalxe2x80x9d stations, having local broadcasting towers. It is almost always the case that different cities in the country have their own lineup of stations due to the localized nature of most television and radio broadcasts. For example, the television station on xe2x80x9cchannel 11xe2x80x9d in Los Angeles is almost certainly different from the television station on xe2x80x9cchannel 11 xe2x80x9d in New York. Due to this phenomenon, it is useful to have systems that use fixed bandwidths for different stations. Therefore, a television set will be able to pick up channel 11 in Los Angeles as easily as picking up channel 11 in New York.
With the advent of satellite television, it is now possible to have a single lineup of stations, all broadcast from a satellite in geosynchronous orbit over the country. This allows the provider to have variable bandwidth channels, such that channel 11, for example, could have a smaller bandwidth than channel 10. Of course, it is still possible to have these variable bandwidth channel systems for use with local television systems, but given the limited signal quality of analog transmissions such a complex system would have little benefit.
FIG. 2 represents an example of a variable bandwidth system. In the example, channel 11 has a smaller bandwidth than channel 10. Channel 11, therefore, would probably have been designated such a small bandwidth because the programming found on that channel is that in which the highest quality sound and/or picture is not required.
A problem arises, however, in filtering variable bandwidth signals. The filter or filters used after the signal has passed through the tuner must be able to filter out different sized bandwidth signals. For example, if the desired channel had a bandwidth of 2 MHz, the signal would have to be passed through a filter designed to filter only 2 MHz xe2x80x9cworthxe2x80x9d of signal, while if the desired channel had a bandwidth of 10 MHz would have to be passed through a filter designed to filter out a 10 MHz signal. If the channel is passed through an incorrect filter, the resulting output will either have a portion of the channel cut off, or have portions of surrounding channels included. The most common solution to this problem is to use a plurality of filters. Therefore, for a system which has 50 different possible channel bandwidth sizes, it would contain 50 different filters.
Addition challenges lie in the tuning of specific channels. The tuners for these types of systems work by selecting a certain band from the signal received. However, analog tuner and filtering systems generally can only select wide bands as they are unable to make the precise filtering required to select a signal having a bandwidth as small as, for example, 2 MHz. The result of this is that after a signal is passed through an analog tuning and filtering system, the output is actually multiple channels worth of signal rather than one single channel. This phenomenon is depicted in FIGS. 3-4. Both FIGS. 3 and 4 show examples of how an analog tuner and filtering system actually selects a wide band of signal, resulting in excess channels (or fragments of channels) included in the output signal. In both of the figures, the user has attempted to tune to channel 11, but the output of the tuner is actually channels 11 and 12, and fragments of channels 10 and 13. While channel 11 is near the middle of the tuned portion in both of the figures, there will still be a need to filter out the remaining channels in order to result in only one channel being viewed on the television set.
Another problem with analog tuners and filters is that they have trouble tuning exactly to the frequency that the user wishes to tune. In a signal that was xe2x80x9cperfectly tunedxe2x80x9d, the tuner would return a signal with a bandwidth whose midpoint was exactly in the middle of the channel desired to be tuned. An example of this xe2x80x9cperfectly tunedxe2x80x9d signal is depicted in FIG. 3. As one can see in FIG. 3, the output of the tuner is still a tuned portion containing multiple channels worth of signal, but the midpoint of this tuned portion is precisely in the middle of channel 11.
An xe2x80x9cimperfectly tunedxe2x80x9d signal is depicted in FIG. 4. As one can see in FIG. 4, the midpoint of the tuned portion is not exactly in the middle of channel 11. Unfortunately, due to the limitations of analog tuners, an xe2x80x9cimperfectly tunedxe2x80x9d channel is quite common, and a xe2x80x9cperfectly tunedxe2x80x9d signal is quite rare. Therefore, there is a need to correct for this tuning error so that the result is a xe2x80x9cperfectly tunedxe2x80x9d (or xe2x80x9cfinely tunedxe2x80x9d) signal.
The result of both tuning the signal and filtering the signal before converting the signal is converted to digital form is that the acquisition time for tuning a specific channel is long. Digital tuning and filtering can be performed much faster than analog. An example of this is the acquisition time in the common digital satellite systems on the market (which use completely analog tuning and filtering devices), where the system takes a second to a few seconds to display the channel on the screen after the user has selected a specific channel.
It is an object of the present invention to provide an apparatus which overcomes some of the limitations of the prior art.
Digital interference rejection of a signal is accomplished by first converting the signal to digital. Then a second signal is generated and mixed with the first signal. This combined signal is then filtered. The signal can then be scaled as needed, resulting in a finely tuned, interference free signal.